This project has the long-range goal of prevention by immunological means of naturally occurring or chemically induced tumors in animal model systems that have viral, immunologic and epidemiologic similarities to human tumors. To this end, the project has defined the spontaneous tumor and endogenous type C virus expressions in colonies of untreated aging mice and has developed a number of immunoassays to measure the host's natural and induced immune responses to type C viruses and tumor cells. Certain experimental immunization specimens have been found by this project and other investigators to induce an immune response in the host that is effective in preventing experimentally-induced tumors and type C virus expressions. Therefore, a major emphasis of the project now is to specifically define the effector immune responses that occur and are responsible for the successful prevention of tumors, endogenous virus and cells carrying endogenous virus. To achieve this, additional techniques specifically involving lymphocyte characterization based on receptors, surface markers and antibody dependence are being established. An equivalent emphasis is being placed on the nature and specificity of the target antigens that these immune responses are directed against, especially with regard to the type C virus and its gene products.